116 
FLORIDA STATE GEOLOGICAL SURVEY. 
merited with calcium carbonate mixed with more or less dark colored 
clay. The presence of calcium carbonate may be detected by the effer¬ 
vescence when treated with dilute hydrochloric acid. There is also 
a certain amount of dark colored clayey material which appears in the 
form of a flocculent sediment in water. This material varies from 
light gray to nearly black, and is without doubt organic matter. In 
addition, hydrous iron oxide may be detected, usually in the form of 
a coating about the sand, grains, or as a stain along the cracks and on 
the exposed surfaces of the beds. It is this iron compound which 
sometimes gives the exposed surfaces a rusty color. 
ThicknessThe Choctawhatchee marl attains a thickness of over 
fifty feet in the vicinity of Redbay, Walton County, where it is ex¬ 
posed in some small ravines and on the banks of Mill Creek, near 
Holland P. O., Leon County, its thickness exceeds thirty feet. How¬ 
ever, from observations elsewhere it appears probable that the average 
thickness is not more than twenty-five to thirty feet. 
Physiographic ExpressionThe ChoctawhatEhee marl rises to 
the surface in a belt from six to over twelve miles in width, extending 
from southern Walton County eastward to Leon County. While the 
topography of this area is in part determined by the younger forma¬ 
tions, the influence of the soft marls is seen in the deep narrow valleys. 
The slopes are everywhere steep and the small streams often head 
in springs which emerge near the upper surface of the Choctawhatchee 
marl. North and west of Crestview, the surface is characterized by 
similar narrow valleys, but the marls are so effectually concealed by 
younger sands and clays that where present they probably have very 
little influence upon the configuration of the surface. 
Paleontologic CharactersThe Choctawhatchee marl contains an 
abundant fauna consisting of nearly 200 species. The most numerous 
fossil is a small bivalve, Mactra ( Mulinici ) congest a, and associated 
with it are various other mollusca—one of the most characteristic 
being the gastropod Bcphora quadricost at a. Other prominent fossils 
are: Conus ddversarius, Fusus equalis, Crueibulum constrictum, Pec- 
ten eboreus , Venus rileyi, Area incite, A. idonea, Cardium acute- 
laqueatum, C. robustum, Carditamera arata, and various species of 
Turritella, Dentalium, and others. 
Structure: — The Choctawhatchee marl presents no marked pe¬ 
culiarities of structure. It rises to a height of over fifty feet above 
tide along its inner margin and dips gently seaward beneath the 
younger formations which border the coast. Probably the ’formation 
shared in the slight deformation which occurred during the late 
Tertiary or early Quaternary; but the exposures are so limited that no 
satisfactory evidence of folding was observed. 
